Imagery Is a Valuable Data Collection Tool

For the Las Vegas Wash Coordination Committee (LVWCC), collecting aerial photos every six to eight weeks is an affordable way to build an accurate baseline inventory critical to managing the Las Vegas Wash (see main article, "Viva Las Vegas!").

Initially, historical aerial photos and other data were collected to document changes in the wash. The lack of temporal data (aerial coverage may not exist for a period of five years or more) left unanswered many questions about the processes having the greatest impact on the wash's environment (habitat, erosion, etc.) and when the changes occurred. Reports were often written without proper coverage of the site, rendering an incomplete and generalized picture as to how data related to the surrounding environment.

"Imagery is a valuable data collection tool and creates a valuable snapshot in time," said Art Ehrenberg, LVWCC's GIS/advanced research analyst. "We index all photos in our GIS, filling in the information gap and allowing for more accurate, proactive analysis."

In one monitoring program, an ongoing sediment study is determining the direction and amount of sediment movement affecting erosion control and the vigor of the area's wetlands.

Sediment removed from the wash ends up in Lake Mead, where water elevation has varied from 10 to 20 meters over a 10-year period. When lake levels are high, most sediment is deposited in the upper portion of a delta that is under the lake. In recent years, as lake levels dropped, aerial photos documented the reworking of sediment and movement of the delta that, when exposed, has little to no vegetation to hold back erosion.

The wetlands in the wash suffered major losses during storm events in the 1980s and late 1990s. Following one particular 1998 storm, a one-mile section of the wash was analyzed using ERDAS IMAGINE software to orthorectify aerial photos taken before and after the storm. Using the IMAGINE Vector Module to overlay two-foot contours existing before the storm, new contours were generated in the changed areas showing erosion of more than 160,000 tons of sediment during that event.

New aerial photos are also being used with more recent ArcInfo data, confirming that the delta extends approximately two miles into Las Vegas Bay and that 11.2 million tons of sediment shifted there from the wash over a 20- to 25-year period.

What are the impacts to the wash and Lake Mead? What are the effects on water quality? Is the delta growing or are lake levels dropping?

According to Ehrenberg, imagery allows the LVWCC to answer such questions "based on detailed information rather than assumptions," and for this reason, "you can never have too much data."